SDH (Synchronous Digital Hierarchy) is internationally standardized for the optical transmission system as a digital hierarchy for multiplexing the conventional service signals. In the United States, SONET (Synchronous Optical Network) similar to SDH has become a de facto standard. An optical transmission system based on the SONET/SDH specification is the mainstream of the current optical transmission system which hitherto has been widely introduced throughout the world.
In recent years, provided wavelength multiplex transmission (WDM) system capable of corresponding to the explosive growth of the Internet traffic, an Optical Transport Network (OTN) (Nonpatent Document 1) is standardized as a platform transparently transmitting various client such as not only SDH/SONET but also ATM, Ethernet (trademark), and thereby likely to become the mainstream of the future optical transmission system.
Similar to SONET/SDH, the bit rate (clock frequency accuracy) in the OTN is defined as ±20 ppm, and the bit rate in the payload region accommodating the client signal is also defined as ±20 ppm. When the clock of the client signal and the clock of the OTN signal (transmission path side) are asynchronous, the difference in the bit rate causes a phase shift between the client clock and the OTN clock, thereby brings about occurrence of a bit slip. In order to prevent this bit slip, stuffing is implemented in the OTN.
FIG. 1 shows a frame structure of the OTN. In an overhead of an Optical Channel Payload Unit (OPU) in the OTU frame are, a stuffing control byte (Justification Control byte), a byte for inserting stuffing byte for a positive stuffing, and a data storing byte for a negative stuffing are defined. If the bit rate of the client signal and the bit rate of the payload region are not identical, positive or negative stuffing corresponding to their difference is implemented and frequency synchronization of the client signal is achieved. With this process, a number of clients can be time-division multiplexed and a clock accuracy can be kept within the clock accuracy of the OTN standard.
The frequency range capable of frequency synchronization of the client signal is determined according to the rate of the stuffing byte to the payload region byte in the frame. In the case of the OTN frame, stuffing byte of 1 byte is defined relative to the OPU payload region of 4×3808 byte, and frequency difference up to the maximum of ±65 ppm can be absorbed. However, considering the frequency accuracy of the OPU payload at ±20 ppm, the client signal up to a frequency accuracy of ±45 ppm can be accommodated without causing a bit slip.    Non-patent Document 1: ITU-T G. 709